1. Field of the Invention
The present invention relates to artificial fusion implants to be placed into the intervertebral space left remaining after the removal of a damaged spinal disc and specifically to the apparatus for and method of, inserting the implants.
2. Description of the Prior Art
For the purpose of achieving long term stability to a segment of injured spine, a fusion (the joining together of two or more bones via a continuous bridge of incorporated bone) may be performed. Well-known to those skilled in such art is the interbody fusion wherein the disc is partially excised and bone placed within that space previously occupied by that disc material (between adjacent vertebrae) for the purpose of restoring a more normal spatial relationship, and to provide for stability; short term by mechanical support, and long term by the permanent cross bonding of bone from vertebra to vertebra. For fusion to occur within the disc space, it is necessary to prepare the vertebrae to be fused by breaking through, or cutting into, the hardened outside plates of bone (the endplates) to allow the interposed bone graft to come into direct contact with the more vascular cancellous (spongy) bone, and to thereby trick the body into attempting to heal this induced, but controlled, "fracturing" by both bone production and the healing of the grafts to both opposed vertebral surfaces such that they become one continuous segment of bone.
The purpose of the present invention is to provide an implant, and the apparatus and method of inserting the implant within the intervertebral space left after the removal of the disc material and permanently eliminate all motion at that location. To do so, the device of the present invention is space occupying within the disc interspace, rigid, self-stabilizing to resist dislodgement, stabilizing to the adjacent spinal vertebrae to eliminate local motion, and able to intrinsically participate in a vertebra to vertebra bony fusion so as to assure the permanency of the result.
At present, following the removal of a damaged disc, either bone or nothing is placed into the remaining space. Placing nothing into this space allows the space to collapse which may result in damage to the nerves; or the space may fill with scar tissue and eventually lead to a reherniation. The use of bone to fill the space is less than optimal in that bone obtained from the patient requires additional surgery and is of limited availability in its most useful form, and if obtained elsewhere, lacks living bone cells, carries a significant risk of infection, and is also limited in supply as it is usually obtained from accident victims. Furthermore, regardless of the source of the bone, it is only marginal structurally and lacks a means to either stabilize itself against dislodgement, or to stabilize the adjacent vertebrae.
a. Prior Art Implants
There have been an extensive number of attempts to develop an acceptable disc prosthesis (an artificial disc). Such devices by design would be used to replace a damaged disc and seek to restore the height of the interspace and to restore the normal motion of that spinal joint. No such device has been found that is medically acceptable. This group of prosthetic or artificial disc replacements, seeking to preserve spinal motion and so are different from the present invention, would include:
U.S. Pat. No. 3,867,728 to STUBSTAD--describing a flexible disc implant.
U.S. Pat. No. 4,349,921 to KUNTZ--describing a flexible disc replacement with file-like surface projections to discourage device dislocation.
U.S. Pat. No. 4,309,777 to PATIL--describing a motion preserving implant with spiked outer surfaces to resist dislocation and containing a series of springs to urge the vertebrae away from each other.
U.S. Pat. No. 3,875,595 to FRONING--describing a motion preserving bladder-like disc replacement with two opposed stud-like projections to resist dislocation.
U.S. Pat. No. 2,372,622 to FASSIO (France)--describing a motion preserving implant comprising complimentary opposed convex and concave surfaces.
In summary, these devices resemble the present invention only in that they are placed within the intervertebral space following the removal of a damaged disc. In that they seek to preserve spinal motion, they are diametrically different from the present invention which seeks to permanently eliminate all motion at that spinal segment.
A second related area of prior art includes those devices utilized to replace essentially wholly removed vertebrae. Such removal is generally necessitated by extensive vertebral fractures, or tumors, and is not associated with the treatment of disc disease. While the present invention is to be placed within the disc space, these other vertebral devices cannot be placed within the disc space as at least one vertebra has already been removed such that there no longer remains a "disc space". Furthermore, these devices are limited in that they seek to perform as temporary structural members mechanically replacing the removed vertebrae (not a removed disc), and do not intrinsically participate in supplying osteogenic material to achieve cross vertebrae bony fusion. Therefore, unlike the present invention which provides for a source of osteogenesis, use of this group of devices must be accompanied by a further surgery consisting of a bone fusion procedure utilizing conventional technique. This group consisting of vertebral struts rather than disc replacements would include the following:
U.S. Pat. No. 4,553,273 to WU--describing a turnbuckle-like vertebral strut.
U.S. Pat. No. 4,401,112 to REZAIAN--describing a turnbuckle-like vertebral strut with the addition of a long stabilizing staple that spans the missing vertebral body.
U.S. Pat. No. 4,554,914 to KAPP--describing a large distractible spike that elongates with a screw mechanism to span the gap left by the removal of an entire vertebra and to serve as an anchor for acrylic cement which is then used to replace the missing bone (vertebrae).
U.S. Pat. No. 4,636,217 to OGILVIE--describing a vertebral strut mechanism that can be implanted after at least one vertebrae has been removed and consists of a mechanism for causing the engagement of screws into the vertebrae above and the vertebrae below the one removed.
In summary, this second group of devices differs from the present invention in that they are vertebral replacements struts, do not intrinsically participate in the bony fusion, can only be inserted in the limited circumstances where an entire vertebra has been removed from the anterior approach, and are not designed for, or intended to be used for the treatment of disc disease.
A third area of prior art related to the present invention includes all devices designed to be applied to one of the surfaces of the spine. Such devices include all types of plates, struts, and rods which are attached by hooks, wires and screws. These devices differ significantly from the present invention in that they are not inserted within the disc space and furthermore do not intrinsically participate in supplying osteogenic material for the fusion.
Therefore, where permanent spinal immobilization is desired, an additional surgery, consisting of a spinal fusion performed by conventional means or the use of supplemental methylmethacrylate cement is required. Such devices applied to the spine, but not within the disc space, would include the following:
U.S. Pat. No. 4,604,995 to STEPHENS--describing a "U" shaped metal rod attached to the posterior elements of the spine with wires to stabilize the spine over a large number of segments.
U.S. Pat. No. 2,677,369 to KNOWLES--describing a metal column device to be placed posteriorly along the lumbar spine to be held in position by its shape alone and to block pressure across the posterior portions of the spinal column by locking the spine in full flexion thereby shifting the maximum weight back onto the patient's own disc.
Other devices are simply variations on the use of rods (e.g. Harrington, Luque, Cotrel-Dubosset, Zielke), wires or cables (Dwyer), plates and screws (Steffee), or struts (Dunn, Knowles).
In summary, none of these devices are designed to be nor can be used within the disc space. Moreover, these devices do not replace a damaged disc, and do not intrinsically participate in the generation of a bony fusion.
Another area of related prior art to be considered is that of devices designed to be placed within the vertebral interspace following the removal of a damaged disc, and seeking to eliminate further motion at that location.
Such a device is contained in U.S. Pat. No. 4,501,269 issued to BAGBY which describes an implantable device and limited instrumentation. The method employed is as follows: a hole is bored transversely across the joint and a hollow metal basket of larger diameter than the hole is then pounded into the hole and then the hollow metal basket is filled with the bone debris generated by the drilling.
While the present invention (device, instrumentation, and method) may appear to bear some superficial resemblance to the BAGBY invention, it is minimal, while the differences are many fold and highly significant. These differences include the following:
1. Safety--The present invention provides for a system of completely guarded instrumentation so that all contiguous vital structures (e.g. large blood vessels, neural structures) are absolutely protected. The instrumentation of the present invention also makes overpenetration by the drill impossible. Such overpenetration in the cervical spine, for example, would result in the total paralysis or death of the patient. In the thoracic spine, the result would be complete paraplegia. In the lumbar spine, the result would be paraplegia or a life-threatening perforation of the aorta, vena cava, or iliac vessels.
The present invention is atraumatically screwed into place while the BAGBY device, in contradistinction, is pounded into position. BAGBY describes that its implant is significantly larger in size than the hole drilled and must be pounded in. This is extremely dangerous and the pounding occurs directly over the spinal cord which is precariously vulnerable to percussive injury. Furthermore, while it is possible, for example in the lumbar spine, to insert the present invention away from the spinal cord and nerves, the BAGBY device must always be pounded directly towards the spinal cord.
Furthermore, since the BAGBY device is pounded into a smooth hole under great resistance, and lacking any specific design features to secure it, the device is highly susceptible to forceful ejection which would result in great danger to the patient and clinical failure. The present invention, in contradistinction, is securely screwed into place, and possesses highly specialized locking threads to make accidental dislodgement impossible. Because of the proximity of the spinal cord, spinal nerves, and blood vessels, any implant dislodgement as might occur with the BAGBY device might have catastrophic consequences.
2. Broad applicability--The BAGBY device can only be inserted from the front of the vertebral column, however, in contrast, the present invention can be utilized in the cervical, thoracic, and lumbar spine, and can be inserted from behind (posteriorly) in the lumbar spine. This is of great importance in that the purpose of these devices is in the treatment of disc disease and probably greater than 99 percent of all lumbar operations for the treatment of disc disease are performed from behind where the present invention can easily be utilized, but the BAGBY device, as per BAGBY'S description, cannot.
3. Disc removal--The BAGBY invention requires the complete removal of the disc prior to the drilling step, whereas the present invention eliminates the laborious separate process of disc removal and efficiently removes the disc and prepares the vertebral end plates in a single step.
4. Time required--The present invention saves time over the BAGBY invention since time is not wasted laboring to remove the disc prior to initiating the fusion. Also, with the present invention the procedure is performed through a system of guarded instrumentation, time is not wasted constantly placing and replacing various soft tissue retractors throughout the procedure.
5. Implant stability--Dislodgement of the implant would be a major source of device failure (an unsuccessful clinical result), and might result in patient paralysis or even death. As discussed, the BAGBY device lacks any specific means of achieving stability and since it is pounded in against resistance to achieve vertebral distraction, and is susceptible to forceful dislodgement by the tendency of the two distracted vertebrae, to return to their original positions squeezing out the device. The present invention, however, is screwed into place. As there is no unscrewing force present between the vertebrae, compression alone cannot dislodge the implant. The implant is inherently stable by its design. Furthermore, the threads of the present invention are highly specialized in that they are periodically interrupted so that the tail ends of each of the tabs so formed are blunted and twisted so as to resist accidental unscrewing. The removal of an implant with such "locking threads" requires the use of a special extractor included within the instrumentation. The stability of the present invention is still further enhanced, again in contradistinction to the BAGBY device, by the presence of a "bone ingrowth" surface texturing, which both increases the friction of the fit and allows for the direct growth of the vertebral bone into the casing of the implant itself.
6. Spinal stability--The present invention is not only self-stabilizing, it also provides stability to the adjacent vertebrae in at least three ways that the BAGBY device cannot. First, the BAGBY device is placed transversely across the joint in the center, leaving both vertebrae free to rock back and forth over this round barrel shaped axis, much like a board over a barrel, being used for a seesaw.
Secondly, as the BAGBY device lacks any specific design features to resist sliding, it may actually behave as a third body allowing the translation of the vertebrae relative to the device and to each other.
Thirdly, any device can only provide stability if it remains properly, seated. The present invention is inherently stable, and therefore assures that it will stabilize the adjacent vertebrae, rather than, as with the BAGBY, the instability of the spine to be treated may cause a dislocation of the BAGBY implant, with further loss of spinal stability.
7. The collapse of the interspace--While both the present invention and the BAGBY device can be fabricated to withstand the compression forces within the interspace, the interspace may nevertheless collapse under the superincumbent body weight as the implant settles into the vertebral bone. This is related to the load per unit area. Again the present invention is superior to the BAGBY device in at least four ways.
First, the present invention offers considerably greater surface area to distribute the load. Secondly, while the BAGBY device is placed centrally, the present device is placed bilaterally where the bone tends to be more cortical and much stronger out towards the rim. Thirdly, the present invention supports the load achieving an "I" beam effect, whereas the BAGBY implant does not. Fourthly, it is not pressure alone that causes the collapse of the bone adjacent to the implant, but also bony erosion that is caused by the motion under pressure of the implant against the bone. As discussed in item 6 above, the present invention alone is highly resistant to such motion, again diminishing the likelihood of erosion and interspace collapse.
8. Bone ingrowth surface texturing--The present invention has a surface treatment of known and conventional technology to induce the growth of bone from the vertebrae directly into the casing material of the implant itself. The BAGBY device has no similar feature.
9. Fusion mass--The BAGBY invention calls for removing the disc and then drilling a hole between the adjacent vertebrae. The bony debris so generated is then put into the device. The present invention takes a core of pure bone producing marrow from the iliac crest, and then by use of a special press, forcibly injects the implant device with an extremely dense compressed core of that osteogenic material until the material itself virtually extrudes from every cell of the implant.
10. The probability of achieving fusion--The fusion rate within the spine is known to be related directly to the amount of exposed vascular bone bed area, the quality and quantity of the fusion mass available, and the extent of the stabilization obtained with all other factors being half constant. It would then be anticipated, that the fusion rate would be superior with the present invention as compared to the BAGBY device, because of optimal implant stability (#5), optimal spinal stability (#6), bone ingrowth surface treatment (#8), superior fusion mass (#9), and the greater exposed vertebral bony surface area (#7).
The last area of prior art possibly related to the present invention and therefore, to be considered related to "bony ingrowth", are patents that either describe methods of producing materials and or materials or devices to achieve the same. Such patents would include:
U.S. Pat. No. 4,636,526 (DORMAN), U.S. Pat. No. 4,634,720 (DORMAN), U.S. Pat. No. 4,542,539 (ROWE), U.S. Pat. No. 4,405,319 (COSENTINO), U.S. Pat. No. 4,439,152 (SMALL), U.S. Pat. No. 4,168,326 (BROEMER), U.S. Pat. No. 4,535,485 (ASHMAN), U.S. Pat. No. 3,987,499 (SCHARBACH), U.S. Pat. No. 3,605,123 (HAHN), U.S. Pat. No. 4,655,777 (DUNN), U.S. Pat. No. 4,645,503 (LIN), U.S. Pat. No. 4,547,390 (ASHMA), U.S. Pat. No. 4,608,052 (VAN KAMPEN), U.S. Pat. No. 4,698,375 (DORMAN), U.S. Pat. No. 4,661,536 (DORMAN), U.S. Pat. No. 3,952,334 (BOKROS), U.S. Pat. No. 3,905,047 (LONG), U.S. Pat. No. 4,693,721 (DUCHEYNE), U.S. Pat. No. 4,070,514 (ENTHERLY).
However, while the implant of the present invention would utilize bone ingrowth technology, it would do so with conventional technology.
b. Prior Art Instrumentations And Methods
The following is a history of the prior art apparatus and methods of inserting spinal implants:
In 1956, Ralph Cloward developed a method and instruments which he later described for preparing the anterior aspect (front) of the cervical spine, and then fusing it. Cloward surgically removed the disc to be fused across and then placed a rigid drill guide with a large foot plate and prongs down over an aligner rod and embedded said prongs into the adjacent vertebrae to maintain the alignment so as to facilitate the reaming out of the bone adjacent the disc spaces. As the large foot plate sat against the front of the spine, it also served as a fixed reference point to control the depth of drilling. The reaming left two opposed resected arcs, one each, from the opposed vertebral surfaces. The tubular drill guide, which was placed only preliminary to the drilling, was thereafter completely removed. A cylindrical bony dowel, significantly larger in diameter than the hole formed, was then pounded into the hole already drilled. Cloward's method of instrumentation was designed for, and limited to, use on the anterior aspect and in the region of the cervical spine only. The hole was midline, which would preclude its use posteriorly where the spinal cord would be in the way.
As the bone graft to be inserted in Cloward's method was necessarily larger in diameter than the hole drilled, the graft could not be inserted through the drill guide. This mandated the removal of the drill guide and left the graft insertion phase completely unprotected. Thus Cloward's method and instrumentation was inappropriate for posterior application.
In addition, the failure to provide continuous protection to the delicate neural structures from the instruments, as well as the bony and cartilaginous debris generated during the procedure, made Cloward's method inappropriate for posterior application. Also, the drill guide described by Cloward could not be placed posteriorly within the spinal canal, as the foot plate would crush the nerves. Modifying Cloward's drill guide by removing the foot plate completely, would still leave the instrument unworkable as it would then lack stability, and would not be controllable for depth of seating.
Nevertheless, Wilterberger, (Wilterberger, B. R., Abbott, K. H., "Dowel Intervertebral Fusion as Used in Lumbar Disc Surgery," The Journal of Bone and Joint Surgery, Volume 39A, pg. 234-292, 1957) described the unprotected drilling of a hole from the posterior into the lumbar spine between the nerve roots and across the disc space, and then inserting a stack of button-like dowels into that space. While Wilterberger had taken the Cloward concept of circular drilling and dowel fusion and applied it to the lumbar spine from a posterior approach, he had not provided for an improved method, nor had he advanced the instrumentation so as to make that procedure sufficiently safe, and it rapidly fell into disrepute.
Crock (Crock, H. V., "Anterior Lumbar Interbody Fusion Indications for its Use and notes on Surgical Technique," Clinical Orthopedics, Volume 165, pg. 157-163, 1981) described his technique and instrumentation for Anterior Interbody Fusion of the lumbar spine, wherein he drilled two large holes side by side across the disc space from anterior to posterior essentially unprotected and then pounded in two at least partially cylindrical grafts larger than the holes prepared.
A review of the prior art is instructive as to a number of significant deficiencies in regard to the method and instrumentation for the performance of Interbody Spinal Fusion utilizing drilling to prepare the endplates.
As the great majority of spinal surgery is performed in the lumbar spine and from posteriorly, a review of the prior art reveals a number of deficiencies in regard to the spine in general, and to the posterior approach to the lumbar spine specifically. These deficiencies include the:
1. Failure to protect the surrounding tissues throughout the procedure, specifically, prior to drilling and until after the insertion of the graft; PA1 2. Failure to contain the debris, bony and cartilaginous, generated during the procedure; PA1 3. Failure to optimize the contact of the cylindrical drill hole and bone graft, the mismatch in their diameters resulting in incongruence of fit; PA1 4. Failure to determine the optimal drill size prior to drilling; PA1 5. Failure to determine the optimal amount of distraction prior to drilling; PA1 6. Inability to optimize the amount of distraction so as to restore the normal spatial relationships between adjacent vertebrae; PA1 7. Inability to create sufficient working space within the spinal canal (between the nerve roots and the dural sac) to make the procedure safe; PA1 8. Absent a foot plate on the drill guide, as necessitated by the close tolerances posteriorly, the inability to reliably insure that the drilling is parallel to the vertebral endplates; PA1 9. The inability to insure equal bone removal from the opposed vertebral surfaces; and PA1 10. The inability to determine within the spinal canal, the proper side by side positioning for dual drill holes.